At 12+ in, I’d be hesitant to put one in a 4 foot tank.
With the exception of eel-like fishes, I wouldn’t do anything that gets past 8 inches; 10 for something really inactive.
Plenty of polys are good options, but they aren’t super active except at feeding time. Then again, most predators aren’t.
If you’re looking for a really active predator, look into a shoal of exodon tetras. They are basically 3 inch piranhas.